I'm not a huge fan of the U.S. Patent Classification System. It's not intuitive and its documentation can be intimidating. The structure of the USPC is a mish-mash of historical practices and applied theories. Although it's frequently described as a hierarchical system, it's actually flat, like a train. The numbering and arrangement of the classes (railway cars) has no relationship to their contents. For example, patents relating to Fences, Active Solid-State Devices and Railway Mail Delivery are classified in Classes 256, 257 and 258, respectively. When the number of documents in a class becomes to large, the USPTO simply moves the overflow to another class, not necessarily next to the original. For example, Class 128: Surgery is continued in Classes 604, 606 and 607. All four classes are treated as one mega-class.
As much as I dislike the USPC, it has some useful features (hidden gems) that are worth mentioning. For example, the special classifications called cross-reference art collections (CRACs) and digests. Digests and CRACs differ slightly in scope, but their functions are similar: to identify specific subject matter that doesn't quite fit into an existing USPC subclass. CRACs and digests are located at the end of the class schedule; numbers 900-999 are reserved for CRACs, while digests are identified by the prefix DIG. For example, in Class 2: Apparel, subclass 901 is used for articles of clothing with antibacterial, antitoxin or clean room properties. In the same class, DIG10 is reserved for inflatable hats. Neither code can be assigned as the primary classification.
These supplemental classifications are extremely useful for searching concepts that are not provided for in the regular classification system, e.g. drugs for the treatment of specific types of diseases, chemical technology for decreasing pollution, etc.